Power control circuits and methods

ABSTRACT

Power control circuits and methods are disclosed, suitable for a power supplier. A power control circuit has a clock generator, a phase controller and a power limiter. The clock generator provides a clock signal, substantially determining switching cycles of a power supply. The phase controller outputs a burst signal based on a group reference signal and a burst initiation signal, and makes a burst period corresponding to a burst signal not less than a group reference period corresponding to the group reference signal. The burst signal is capable of switching the power supplier between a switching state and a non-switching state. The power limiter limits the power transferred by the power supply in every switching cycle, during a burst-up duration after the power supply is switched from the non-switching state to the switching state. The burst initiation signal correlates to an output voltage of the power supplier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to control methods of a power supply, and more particularly relates to control methods when operating under light load or no load conditions of the power supply.

2. Description of the Prior Art

A power supply is used to convert power to within a specific level to power electronic devices or components. The power consumed by the power supply should be as low as possible to improve conversion efficiency. Even small power consumption will decrease conversion efficiency by a great deal, particularly under light load or no load conditions of the power supply. Consequently, a major consideration of designing the power supply is to lower power consumption for light load or no load conditions of the power supply.

A switching mode power supply is designed to operate in skip mode or burst mode under light load or no load conditions. FIG. 1 illustrates a flyback power supply 60 of the prior art. A power management controller 74 controls a power switch 72 to store power from AC input or deliver power to output through a transformer 65. A compensation signal S_(COM) is controlled by an output voltage V_(OUT) through a feedback loop comprising LT431 and a photo coupler 63. FIG. 2 is a simplified block diagram illustrating a power management controller 74 a of the prior art according to one embodiment of FIG. 1. When a burst signal S_(BST) is asserted, a clock generator 86 generates a clock signal S_(CLK) to periodically switch the power switch 72 on and off by a logic controller 62; this is referred to as a switching state. The clock signal S_(CLK) substantially defines switching cycles of the power supply 60. On time of the power switch 72 is controlled by a limiting signal S_(CS-L) and a comparator 82. The limiting signal S_(CS-L) is generated by a level shifter 67, and can be regarded as equivalent to the compensation signal S_(COM). A resistor 61 forms a path to a power source Vcc for the compensation signal S_(COM). When the compensation signal S_(COM) is lower than a burst reference voltage V_(BST-REF), a comparator 84 sets a burst signal S_(BST) to logic 0, hence turning off the power switch 72 regardless of the clock signal S_(CLK), and maintaining the power switch 72 in a non-switching state. FIG. 3 illustrates possible waveforms of signals S_(CLK), V_(G), V_(CS) of the power management controller 74 a of FIG. 2 operating under light load or no load. As the compensation signal S_(COM) varies, a control signal V_(G) of the power management controller 74 a forces the power switch 72 to conduct for one or consecutive switching cycles, then turns off the power switch 72 for the following one or consecutive switching cycles. The above described mode is referred to as skip mode or burst mode, and is referred to as burst mode hereinafter.

Burst mode is dedicated to stopping consecutive mostly ineffective switching cycles and focuses power conversion on consecutive more effective switching cycles. Burst mode can cause annoying audible noise without proper control of power conversion in effective switching cycles. For instance, audible noise occurs if the sum of the switching state period T_(B) and the non-switching state period T_(S), period T_(G), corresponding to frequency f_(G), falls within the audible frequency range.

SUMMARY OF THE INVENTION

The embodiment of the present invention presents a power control circuit adapted to a power supply. The power control circuit comprises a clock generator, a phase controller, a power limiter. The clock generator provides a clock signal to substantially determine switching cycles of the power supply. The phase controller generates a burst signal having a period not smaller than a period of a group reference signal according to the group reference signal and a burst initiation signal. The burst signal switches the power supply to either a switching state or a non-switching state. The power limiter limits a power output of the power supply in every switching cycle during a burst up duration after the power supply switches from the non-switching state to the switching state. The burst up duration is not smaller than the period of the group reference signal. The burst initiation signal relates to a power output of the power supply.

The embodiment of the present invention also extends to a power control method adapted to a power supply, comprising: generating a burst initiation signal related to a power output of the power supply; generating a burst signal having a period not smaller than a period of a group reference signal according to the burst initiation signal and the group reference signal, wherein the burst signal can switch the power supply to either a non-switching state or a switching state; limiting the power output of the power supply in every switching cycle within a burst up duration after the power supply switches from the non-switching state to the switching state, wherein the burst up duration is not smaller than the period of the group reference signal.

The embodiment of the present invention extends further to a power control circuit adapted to a power supply, comprising: a clock generator, a group reference signal generator, a burst timer, and a power limiter. The clock generator provides a clock signal for substantially determining switching cycles of the power supply. The group reference signal generator generates a group reference signal having a period longer than a switching cycle of the clock signal. The burst timer determines a burst up duration after the power supply switches from a non-switching state to a switching state, wherein the burst up duration is not smaller than the period of the group reference signal. The power limiter limits a power output of the power supply in every switching cycle within the burst up duration.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a flyback power supply of the prior art.

FIG. 2 is a diagram illustrating a power management controller of the prior art.

FIG. 3 is a diagram illustrating possible signal waveforms of the power management controller of FIG. 2 operating under light load or no load.

FIG. 4 is a diagram illustrating a power management controller according to the embodiment of the power supply of FIG. 1.

FIG. 5 is a diagram illustrating the phase controller and the peak limiter of FIG. 4.

FIG. 6 is a diagram illustrating possible signal waveforms under different load conditions of the power management controller of FIG. 4.

FIG. 7 is a diagram illustrating a power management controller adapted to the power supply of FIG. 1 of the present invention.

FIG. 8 is a diagram illustrating signal waveforms related to a clock generator of FIG. 7.

FIG. 9 is a diagram illustrating a power management controller adapted to the power supply of FIG. 1 of the present invention.

FIG. 10 is a diagram illustrating the loop compensation controller of FIG. 9.

FIG. 11 is a diagram illustrating a power management controller adapted to the present invention.

FIG. 12 is a diagram illustrating a power management controller adapted to the power supply of FIG. 1 of the present invention.

FIG. 13 is a diagram illustrating a phase controller of FIG. 12.

FIG. 14 is a diagram illustrating a power management controller adapted to the power supply of FIG. 1 of the present invention.

FIG. 15 is a diagram illustrating the phase controller and the peak limiter of FIG. 14.

FIG. 16 is a diagram illustrating a burst timer of FIG. 15.

FIG. 17 is a diagram illustrating timing diagram of certain signals of FIG. 16 and FIG. 14.

FIG. 18 is a diagram illustrating a group reference signal generator of FIG. 15.

FIG. 19 is a diagram illustrating timing diagram of certain signals of FIG. 18.

DETAILED DESCRIPTION

Embodiments of the present invention are illustrated by a flyback (converter) power supply, but the invention is not limited thereto, and may be adapted to, boost, buck, or other converter topologies.

FIG. 4 is a simplified block diagram illustrating a power management controller 74 b according to the embodiment of the power supply 60 of FIG. 1. The power management controller 74 b can limit group frequency f_(G) to lower than a certain value. For instance, group frequency f_(G) is lower than 1 kHz to reduce audible noise within harmonic frequencies of f_(G) between 5 kHz and 15 kHz range.

Comparing FIG. 4 and FIG. 2, FIG. 4 comprises a phase controller 64, a peak limiter 66, and an exit comparator 68. Other components are well known to those of ordinary skill in the art.

The phase controller 64 comprises three input terminals, individually receiving a clock signal S_(CLK) (from a clock generator 86, a burst initiation signal S_(BST-INN) from a comparator 84, and an exit signal S_(EXT) from an exit comparator 68, and hence generating a burst signal S_(BST) and a suppression signal S_(DPS). The power supply 60 should switch to a non-switching state or to a switching state when the burst initiation signal S_(BST-INN) changes, but the phase controller 64 may or may not change the state of the burst signal S_(BST) instantly. The state of the burst signal S_(BST) is changed according to a phase difference of the burst initiation signal S_(BST-INN) and a group reference signal S_(SCLK). Under some circumstances, the phase controller 64 may enable the suppression signal S_(DPS) to limit the peak of a current sense signal V_(CS) to be a certain value and to not vary with a compensation signal S_(COM) by the peak limiter 66. Detailed illustration of the phase controller 64 and the peak limiter 66 will be shown in a later section.

FIG. 5 illustrates a simplified block diagram of the phase controller 64 and the peak limiter 66 of FIG. 4.

The phase controller 64 comprises a frequency divider 28, a suppression signal generator 26, and a frequency limiter 24. The frequency divider 28 generates the group reference signal S_(SCLK) with a frequency lower than the clock signal S_(CLK) according to the clock signal S_(CLK). For instance, the frequency of the group reference signal S_(SCLK) is set to 1 kHz when the frequency of the clock signal S_(CLK) is 25 kHz. The suppression signal generator 26 sets the suppression signal S_(DPS) to logic 0 when a release signal S_(FREE) or the exit signal S_(EXT) is logic 1.

The frequency limiter 24 limits the frequency of the burst signal S_(BST) to be lower than the frequency of the group reference signal S_(SCLK). The frequency limiter 24 comprises a phase comparator 22 to compare the phase difference of the group reference signal S_(SCLK) and the burst initiation signal S_(BST-INN). In FIG. 5, the phase comparator 22 compares the time difference of the rising edge of the group reference signal S_(SCLK) and the rising edge of the burst initiation signal S_(BST-INN). If the group reference signal S_(SCLK) rises earlier, the release signal S_(FREE) is asserted (logic 1). On the contrary, if the burst initiation signal S_(BST-INN) rises earlier, a standby signal S_(STD) is asserted.

FIG. 6 illustrates possible signal waveforms under different load conditions of the power management controller 74 b of FIG. 4 to explain the operation of circuit of FIG. 5. From top to bottom, FIG. 6 shows the current sense signal V_(CS), the clock signal S_(CLK), the group reference signal S_(SCLK), the compensation signal S_(COM), the burst initiation signal S_(BST-INN), the release signal S_(FREE), the standby signal S_(STD), a reset signal S_(R), the suppression signal S_(DPS), and the burst signal S_(BST). Other waveforms can be inferred from the assumed waveform of the compensation signal S_(COM), of FIG. 6 with reference to the circuit of FIG. 1, FIG. 4, and FIG. 5.

As illustrated in group period T_(G1) from t₁ to t₃, even though the compensation signal S_(COM) has risen above a burst reference voltage V_(BST-REF), the burst signal S_(BST) does not rise, and the power supply 60 remains in the non-switching state until the reference clock signal S_(SCLK) rises at t₃. The group frequency (inverse of group period T_(G1)) is equal to the frequency of the group reference signal S_(SCLK).

As illustrated in group period T_(G2) from t₃ to t₅, due to the release signal S_(FREE) having risen at t₄, the burst signal S_(BST) rises as the compensation signal S_(COM) rises above the burst reference voltage V_(BST-REF). The group frequency (inverse of group period T_(G2)) is lower than the frequency of the group reference signal S_(SCLK).

Concluding from group period T_(G1) and T_(G2), the burst signal S_(BST) does not rise before the rising edge of the group reference signal S_(SCLK). Therefore, the burst signal S_(BST) corresponds to a group frequency not higher than the frequency of the group reference signal S_(SCLK).

As illustrated, at t₁ and t₃, peaks of the current sense signal V_(CS) remain at a fixed value and do not vary with the compensation signal S_(COM), even though the power supply 60 is in the switching state. The reason is that whenever the burst signal S_(BST) is asserted, the reset signal S_(R) is also asserted, thereby disasserting the release signal S_(FREE) and asserting the suppression signal S_(DPS), which causes an input signal S_(COMSEL) of level shifter 67 to be fixed at a reference voltage V_(DPS-REF). At t₄, the release signal S_(FREE) is asserted, so the input signal S_(COMSEL) of level shifter 67 becomes the compensation signal S_(COM) coming from the peak limiter 66, and peaks of the current sense signal V_(CS) vary with the compensation signal S_(COM).

In FIG. 4, the power supply 60 should enter the switching state under heavy load conditions instantly when the compensation signal S_(COM) is above an exit reference voltage V_(EXT-REF). For instance, the burst reference voltage V_(BST-REF) is 2V and the exit reference voltage V_(EXT-REF) is 3V. In FIG. 5, when the exit signal S_(EXT) is asserted, the burst signal S_(BST) is also asserted, and the compensation signal S_(COM) of the peak limiter 66 becomes the input signal S_(COMSEL).

FIG. 7 is one embodiment of a power management controller 74 c adapted to the power supply 60 of FIG. 1 of the present invention. FIG. 7 is similar to FIG. 4, the difference being that the clock generator 86 a in FIG. 7 receives the burst initiation signal S_(BST-INN). The clock generator 86 a generates the clock signal S_(CLK) with its frequency varying with the state of the burst initiation signal S_(BST-INN). FIG. 8 shows waveforms of the clock generator 86 a of FIG. 7. When the burst initiation signal S_(BST-INN) is asserted, the clock signal S_(CLK) is at a normal frequency higher than a reduced frequency when the burst initiation signal S_(BST-INN) is disasserted. The function is achieved by varying the slope of a ramp signal V_(RMP) as shown in FIG. 8. In another embodiment, the clock generator generates the clock signal S_(CLK), with its frequency varying with the state of the burst signal S_(BST). One of the advantages of the design is to save power in the non-switching state. Another advantage is to reduce noise in burst mode. When in burst mode, the frequency of the group reference signal S_(SCLK) may not be a fixed value, but depends on the burst initiation signal S_(BST-INN) or the disasserted duration of the burst signal S_(BST). Therefore the group frequency may jitter and disperse audio energy.

FIG. 9 is a power management controller 74 d adapted to the power supply 60 of FIG. 1 of the present invention. FIG. 9 is similar to FIG. 4, the difference being an additional loop compensation controller 69 connected between a resistor 61 and a power source Vcc. FIG. 10 illustrates the loop compensation controller 69 of FIG. 9. When the burst initiation signal S_(BST-INN) is asserted, a switch 32 is short circuited. The short circuited switch 32 provides the power source Vcc to the compensation signal S_(COM). When the burst initiation signal S_(BST-INN) is disasserted, and the power supply 60 operates in the non-switching state, a divider 30 enables clock signal S_(CLK) to short circuit the switch 32 for one switching cycle once every four switching cycles, and the switch 32 remains open circuited for the rest of the time. In so doing, more power can be saved in the non-switching state.

FIG. 11 is a power management controller 74 e adapted to the present invention. FIG. 11 integrates parts of circuits in FIG. 4, FIG. 7, and FIG. 9. Operation of the circuit in FIG. 11 can be inferred from the illustration and explanation of FIG. 4, FIG. 7, and FIG. 9.

FIG. 12 is a power management controller 74 f adapted to the power supply 60 of FIG. 1 of the present invention. FIG. 13 illustrates a phase controller 70 of FIG. 12. The phase controller 64 compares the rising edges of the group reference signal S_(SCLK) and the burst initiation signal S_(BST-INN). The phase controller 70 of FIG. 13 comprises inverters 42 and 44. The phase controller compares the falling edges of the group reference signal S_(SCLK) and the burst initiation signal S_(BST-INN). The phase controller 70 limits the frequency of two consecutive falling edges of the burst signal S_(BST) to be smaller than the frequency of the group reference signal S_(SCLK).

FIG. 14 is a power management controller 74 g adapted to the power supply 60 of FIG. 1 of the present invention. The burst signal S_(BST) switches the power supply 60 between the switching state and the non-switching state. FIG. 14 is similar to the embodiments hereinbefore. A phase controller 64 a in FIG. 14 provides the group reference signal S_(SCLK) and limits the group period corresponding to the burst signal S_(BST) to be not smaller than the period of the group reference signal S_(SCLK). The period indicates the duration between two consecutive rising edges or falling edges of the corresponding signal. The phase controller 64 a also generates the suppression signal S_(DPS) to set a peak limit signal V_(CS-LIMIT) of a peak limiter 66 a. A comparator 83 limits the current sense signal V_(CS) to be smaller than the peak limit signal V_(CS-LIMIT) in every switching cycle (equivalent to the cycle of the clock signal S_(CLK)). So, the peak limiter 66 a and the comparator 83 can be regarded as a power limiter, limiting the power conversion of the power supply 60 in every switching cycle.

FIG. 15 illustrates the phase controller 64 a and the peak limiter 66 a of FIG. 14. Simply said, the phase controller 64 a controls the group period and the peak limiter 66 a controls the power output in every switching cycle. Audible noise can be reduced by proper adjustment of the said controller and limiter.

The phase controller 64 a comprises the reference generator 33, a suppression signal generator 26 a, a burst timer 35, and the frequency limiter 24. The group reference signal generator 33 generates the group reference signal S_(SCLK) with a frequency lower than the frequency of the clock signal S_(CLK), which is explained in detail later. The frequency limiter 24 limits the group period corresponding to the burst signal S_(BST) to be not lower than the period of the group reference signal S_(SCLK). The burst timer 35 determines the burst up duration when the power supply 60 switches from the non-switching state to the switching state. A release signal S_(DPSX) is disasserted (logic 0) within the burst up duration. If the exit signal S_(EXT) remains at logic 0 within the burst up duration, the suppression signal S_(DPS) remains at logic 1 and forces the peak limiter 66 a to select a peak limit value V_(CS-LIMIT-L), thus suppressing the power output to a very low level in every switching cycle. For instance, assumes the peak limit value V_(CS-LIMIT-L) is 0.2V, if the suppression signal S_(DPS) is logic 0, the peak limiter 66 a selects a higher peak limit value V_(CS-LIMIT-H). If the peak limit value V_(CS-LIMIT-H) is 0.85V, and under normally heavy load conditions, the current sense signal V_(CS) is limited by the compensation signal S_(COM) and does not reach the peak limit value of V_(CS-LIMIT-H).

FIG. 16 illustrates one embodiment of a burst timer 35. FIG. 17 illustrates some signal waveforms of FIG. 16 and FIG. 14. Assumes a plurality of outputs Q of D flip-flops 90, 92, 94, and 96 are initially logic 0. Only when the power supply 60 switches from the non-switching state to the switching state, and thereby asserts the burst signal S_(BST) to logic 1, can the output Q of D flip-flop 96 being asserted to logic 1, and thereby transmits the group reference signal S_(SCLK) to a plurality of clock inputs of D flip-flops 90, 92, and 94. D flip-flops 90, 92, and 94 can be regarded as a shift register, and the output Q is asserted to logic 1 in sequence according to the number of rising edges of the group reference signal S_(SCLK). For instance, when the burst signal S_(BST) changes from logic 0 to logic 1 and the group reference signal S_(SCLK) rises for the first time, signal Q1 changes from logic 0 to logic 1; when the group reference signal S_(SCLK) rises for the second time, signal Q2 changes from logic 0 to logic 1, as illustrated in FIG. 17. In FIG. 16, when signal Q2 changes from logic 0 to logic 1, a single pulse generator 98 outputs a short pulse signal S_(PLS) to reset outputs Q of D flip-flops 90, 92, 94, and 96, to logic 0. From FIG. 17, as the burst signal S_(BST) changes from logic 0 to logic 1, the release signal S_(DPSX) changes from logic 1 to logic 0 and remains at logic 0 for over one reference cycle. The release signal S_(DPSX) changes from logic 0 to logic 1 when signal Q2 changes from logic 0 to logic 1. The burst up duration is determined by the duration for which the release signal S_(DPSX) remains 0. FIG. 17 also illustrates that, during the burst up duration, the peak value of the current sense signal V_(CS) is fixed to around 0.2V (peak limit value V_(CS-LIMIT-L)) in every switching cycle. The peak value of the current sense signal V_(CS) varies with the limit signal S_(CS-L) outputted by the level shifter 67 in every switching cycle after the burst up duration.

FIG. 18 illustrates the group reference signal generator 33. FIG. 19 illustrates certain signal waveforms of FIG. 18. A charge and discharge control circuit 52 generates the group reference signal S_(SCLK) according to a charge and discharge time of capacitor 58. In one embodiment, the charge and discharge circuit 52 is in a single chip integrated circuit, and connected to an externally connected capacitor through one pin of the single chip integrated circuit. A counter 54 counts the time T_(sense) required for the terminal voltage V_(CT) of capacitor 58 to climb from initial ground voltage to a predetermined voltage, and a result is indicated by SS. The larger the capacitor 58, the longer T_(sense) is, and therefore the larger SS is. A peak limit selector 56 selects one reference voltage from V_(REF-1), V_(REF-2), V_(REF-3), and V_(REF-4) to be set as the peak limit value V_(CS-LIMIT-L) according to the counting result SS. In other words, the peak limit value V_(CS-LIMIT-L) is determined by proper selection of the externally connected capacitor 58.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

What is claimed is:
 1. A power control circuit for controlling a power supply comprising: a clock generator for providing a clock signal for substantially determining switching cycles of the power supply; a phase controller for generating a burst signal having a period not smaller than a period of a group reference signal according to phases of the group reference signal and phases of a burst initiation signal for switching the power supply to a non-switching state or a switching state; and a power limiter for limiting a power output of the power supply in every switching cycle within a burst up duration not smaller than the period of the group reference signal after the power supply is switched from the non-switching state to the switching state; wherein the burst initiation signal is generated according to the power output of the power supply.
 2. The power control circuit of claim 1, wherein the burst initiation signal is generated according to a compensation signal controlled by the power output of the power supply.
 3. The power control circuit of claim 1, wherein the power supply comprises an inductor, and the power limiter limits a plurality of peaks of a current flowing through the inductor to a substantially predetermined value within the burst up duration.
 4. The power control circuit of claim 1, wherein the power limiter limits the power output of the power supply to not exceeding a predetermined value in every switching cycle, the power control circuit further comprising: a power setting unit for setting the predetermined value according to a detected result related to an electronic component.
 5. The power control circuit of claim 4, wherein the power control circuit is a single chip integrated circuit, the electronic component is an externally coupled capacitor, and the power control circuit further comprises: a group reference signal generator for charging and discharging the externally coupled capacitor for generating the group reference signal.
 6. The power control circuit of claim 4, wherein the power setting unit comprises a counter, and the power setting unit selects the predetermined value from a plurality of predetermined values according to a counting result of the counter.
 7. The power control circuit of claim 1 further comprising: a burst timer for determining the burst up duration.
 8. A power control method for controlling a power supply comprising: generating a burst initiation signal according to a power output of the power supplier; generating a burst signal having a period not smaller than a period of a group reference signal according to the group reference signal and the burst initiation signal, wherein the burst initiation signal switches the power supply to a non-switching state or a switching state; and limiting the power output of the power supply in every switching cycle within a burst up duration not smaller than the period of the group reference signal after the power supply switches from the non-switching state to the switching state.
 9. The power control method of claim 8 further comprising generating the burst initiation signal according to a compensation signal controlled by the power output of the power supply.
 10. The power control method of claim 8 further comprising limiting a plurality of peaks of a current flowing through an inductor to a substantially predetermined value in every switching cycle within the burst up duration.
 11. The power control method of claim 10 further comprising setting the predetermined value according to a detected result related to an electronic component.
 12. The power control method of claim 10 wherein the power control method is adapted to a single chip integrated circuit, the electronic component is an externally coupled capacitor, and the method further comprises charging and discharging the externally coupled capacitor to generate the group reference signal.
 13. The power control method of claim 8 further comprising determining the burst initiation signal according to the group reference signal and the burst signal.
 14. A power control circuit for controlling a power supply, wherein the power supply may be in a non-switching state or a switching state, comprising: a clock generator for providing a clock signal for substantially determining switching cycles of the power supply; a group reference signal generator for generating a group reference signal having a period longer than a switching cycle of the clock signal; a burst timer for determining a burst up duration not smaller than a period of the group reference signal after the power supply switches from the non-switching state to the switching state; and a power limiter for limiting a power output of the power supply in every switching cycle within the burst up duration.
 15. The power control circuit of claim 14, wherein the power limiter limits the power output of the power supply to not exceeding a predetermined value in every switching cycle, the power control circuit further comprising: a power setting unit for setting the predetermined value according to a detected result related to an electronic component.
 16. The power control circuit of claim 15, wherein the power control circuit is a single chip integrated circuit, the electronic component is an externally coupled capacitor, and the group reference signal generator charges and discharges the externally coupled capacitor to generate the group reference signal.
 17. The power control circuit of claim 15, wherein the power setting unit comprises a counter, and the power setting unit selects the predetermined value from a plurality of predetermined values according to a counting result of the counter. 